The present invention concerns solar cells, particularly regenerative solar cells, and light harvesting arrays useful in such solar cells.
Molecular approaches for converting sunlight to electrical energy have a rich history with measurable xe2x80x9cphotoeffectsxe2x80x9d reported as early as 1887 in Vienna (Moser, J. Montash. Chem. 1887, 8, 373.). The most promising designs were explored in considerable detail in the 1970""s (Gerischer, H. Photochem. Photobiol 1972, 16, 243; Gerischer, H. Pure Appl. Chem. 1980, 52, 2649; Gerischer, H.; Willig, F. Top. Curr. Chem. 1976, 61, 31). Two common approaches are shown in FIG. 1, both of which incorporate molecules that selectively absorb sunlight, termed photosensitizers or simply sensitizers (S), covalently bound to conductive electrodes. Light absorption by the sensitizer creates an excited state, S*, that injects an electron into the electrode and then oxidizes a species in solution. The right hand side depicts a simplified photoelectrosynthetic cell. This cell produces both electrical power and chemical products. Many of the molecular approaches over the past few decades were designed to operate in the manner shown with the goal of splitting water into hydrogen and oxygen. Shown on the left hand side is a regenerative cell that converts light into electricity with no net chemistry. In the regenerative solar cell shown, the oxidation reactions that take place at the photoanode are reversed at the dark cathode.
The principal difficulty with these solar cell designs is that a monolayer of a molecular sensitizer on a flat surface does not absorb a significant fraction of incident visible light. As a consequence, even if the quantum yields of electron transfer are high on an absorbed photon basis, the solar conversion efficiency will be impractically low because so little light is absorbed. Early researchers recognized this problem and tried to circumvent it by utilizing thick films of sensitizers. This strategy of employing thick absorbing layers was unsuccessful as intermolecular excited-state quenching in the thick sensitizer film decreased the yield of electron injection into the electrode.
One class of thick film sensitizers is provided by the so-called organic solar cells (Tang, C. W. and Albrecht, A. C. J. Chem. Phys. 1975, 63, 953-961). Here a 0.01 to 5 xcexcm thick film, typically comprised of phthalocyanines, perylenes, chlorophylls, porphyrins, or mixtures thereof, is deposited onto an electrode surface and is employed in wet solar cells like those shown, or as solid-state devices where a second metal is deposited on top of the organic film. The organic layer is considered to be a small bandgap semiconductor with either n- or p-type photoconductivity and the proposed light-to-electrical energy conversion mechanisms incorporate excitonic energy transfer among the pigments in the film toward the electrode surface where interfacial electron transfer takes place. However, the importance of these proposed mechanistic steps is not clear. Increased efficiencies that result from vectorial energy transfer among the pigments have not been convincingly demonstrated. Furthermore, the reported excitonic diffusion lengths are short relative to the penetration depth of the light. Accordingly, most of the light is absorbed in a region where the energy cannot be translated to the semiconductor surface. The excitons are also readily quenched by impurities or incorporated solvent, leading to significant challenges in reproducibility and fabrication. The state-of-the-art organic solar cells are multilayer organic xe2x80x9cheterojunctionxe2x80x9d films or doped organic layers that yield xcx9c2% efficiencies under low irradiance, but the efficiency drops markedly as the irradiance approaches that of one sun (Forrest, S. R. et al., J. Appl. Phys. 1989, 183, 307; Schon, J. H. et al., Nature 2000, 403, 408).
Another class of molecular-based solar cells are the so-called photogalvanic cells that were the hallmark molecular level solar energy conversion devices of the 1940""s-1950""s (Albery, W. J. Acc. Chem. Res. 1982, 15, 142). These cells are distinguished from those discussed above in that the excited ""sensitizer does not undergo interfacial electron transfer. The cells often contain sensitizers embedded in a membrane that allows ion transfer and charge transfer; the membrane physically separates two dark metal electrodes and photogenerated redox equivalents. The geometric arrangement precludes direct excited-state electron transfer from a chromophore to or from the electrodes. Rather, intermolecular charge separation occurs and the reducing and oxidizing equivalents diffuse to electrodes where thermal interfacial electron transfer takes place. A transmembrane Nernst potential can be generated by photodriven electron transfer occurring in the membrane. In photoelectrosynthetic galvanic cells, chemical fuels may be formed as well. This general strategy for dye sensitization of electrodes has been employed in many guises over the years, but the absolute efficiencies remain very low. Albery concluded that an efficiency of xcx9c13% theoretically could be achieved in an aqueous regenerative photogalvanic cell. However, efficiencies realized to date are typically less than 2%.
In 1991, a breakthrough was reported by Gratzel and O""Regan (O""Regan, B. et al., J. Phys. Chem. 1990, 94, 8720; O""Regan, B. and Grxc3xa4tzel, M. Nature 1991, 353, 737). By replacing the planar electrodes with a thick porous colloidal semiconductor film, the surface area for sensitizer binding increased by over 1000-fold. Gratzel and O""Regan demonstrated that a monolayer of sensitizer coating the semiconductor particles resulted in absorption of essentially all of the incident light, and incident photon-to-electron energy conversion efficiencies were unity at individual wavelengths of light in regenerative solar cells. Furthermore, a global efficiency of xcx9c5% was realized under air-mass 1.5 illumination conditions; this efficiency has risen to a confirmed 10.69% today (Gratzel, M. in xe2x80x9cFuture Generation Photovoltaic Technologiesxe2x80x9d McConnell, R. D.; AIP Conference Proceedings 404, 1997, page 119). These xe2x80x9cGratzelxe2x80x9d solar cells have already found niche markets and are commercially available in Europe.
These high surface area colloidal semiconductor films (Gratzel cells) achieve a high level of absorption but also have the following significant drawbacks. (1) A liquid junction is required for high efficiency (because the highly irregular surface structure makes deposition of a solid-state conductive layer essentially impossible). (2) The colloidal semiconductor films require high temperature annealing steps to reduce internal resistances. Such high temperatures impose severe limitations on the types of conductive substrates that can be used. For example, polymeric substrates that melt below the required annealing temperatures cannot be used. (3) Significant losses are associated with transporting charge through the thick semiconductor films. These losses do not appreciably decrease the photocurrent, but have a large effect on the voltage output and thus the power is decreased significantly (Hagfeldt, A.; Grxc3xa4tzel, M. Chem. Rev. 1995, 95, 49). Accordingly, there remains a need for new molecular approaches to the construction of solar cells.
Accordingly, the present invention provides, among other things, a light harvesting array useful for the manufacture of solar cells. The light harvesting array comprises:
(a) a first substrate comprising a first electrode; and
(b) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I:                               X          1                ⁢                              xe2x80x94            (                          X                              m                +                1                                      )                    m                                    (        I        )            
xe2x80x83wherein:
m is at least 1, and may be from two, three or four to 20 or more;
X1 is a charge separation group having an excited-state of energy equal to or lower than that of X2; and
X2 through Xm+1 are chromophores.
In light harvesting rods of Formula I herein, X1 preferably comprises a porphyrinic macrocycle, which may be in the form of a double-decker sandwich compound. Further, X2 through Xm+1 also preferably comprise porphyrinic macrocycles.
In one preferred embodiment of the light harvesting rods of Formula I herein, at least one of (e.g., two, three, a plurality of, the majority of or all of) X1 through Xm+1 is/are selected from the group consisting of chlorins, bacteriochlorins, and isobacteriochlorins.
A particular embodiment of a light harvesting array as described above provides for the movement of holes in the opposite direction of excited-state energy along some or all of the length of the light harvesting rods, and comprises:
(a) a first substrate comprising a first electrode; and
(b) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I:                               X          1                ⁢                              xe2x80x94            (                          X                              m                +                1                                      )                    m                                    (        I        )            
xe2x80x83wherein:
m is at least 1 (typically two, three or four to twenty or more);
X1 is a charge separation group having an excited-state of energy equal to or lower than that of X2;
X2 through Xm+1 are chromophores; and
X1 through Xm+1 are selected so that, upon injection of either an electron or hole from X1 into the first electrode, the corresponding hole or electron from X1 is transferred to at least X2, and optionally to X3, X4, and all the way through Xm+1. In a currently preferred embodiment, X1 through Xm+1 are selected so that, upon injection of an electron from X1 into the first electrode, the corresponding hole from X1 is transferred to at least X2, and optionally through Xm+1.
Light-harvesting arrays provide intense absorption of light and deliver the resulting excited state to a designated location within the molecular array. There are a variety of applications of light-harvesting arrays. Light-harvesting arrays can be used as components of low-level light detection systems, especially where control is desired over the wavelength of light that is collected. Light-harvesting arrays can be used as input elements in optoelectronic devices, and as an input unit and energy relay system in molecular-based signaling systems. One application of the latter includes use in molecular-based fluorescence sensors. The molecular-based sensor employs a set of probe groups (which bind an analyte) attached to a molecular backbone that undergoes excited-state energy transfer. The binding of a single analyte to any one of the probe groups yields a complex that can quench the excited state that freely migrates along the backbone (i.e., exciton). The quenching phenomenon results in diminished fluorescence from the molecular backbone. Because only one bound analyte can cause the quenching phenomenon, the sensitivity is much higher than if there was a 1:1 ratio of probe groups and fluorescence groups. Previously, such molecular-based fluorescence sensors have employed UV or near-UV absorbing chromophores in the molecular backbone. The light-harvesting arrays described herein are ideally suited as components for a new class of molecular-based fluorescence sensors that absorb (and fluoresce) strongly in the visible and near-infrared region.
A particular application of the light-harvesting arrays described herein is in solar cells. A solar cell as described herein typically comprises:
(a) a first substrate comprising a first electrode;
(b) a second substrate comprising a second electrode, with the first and second substrate being positioned to form a space therebetween, and with at least one of (i) the first substrate and the first electrode and (ii) the second substrate and the second electrode being transparent;
(c) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I:                               X          1                ⁢                              xe2x80x94            (                          X                              m                +                1                                      )                    m                                    (        I        )            
xe2x80x83wherein:
m is at least 1 (and typically two, three or four to twenty or more);
X1 is a charge separation group having an excited-state of energy equal to or lower than that of X2;
X2 through Xm+1 are chromophores; and
X1 is electrically coupled to the first electrode; the solar cell further comprising
(d) an electrolyte in the space between the first and second substrates. A mobile charge carrier can optionally be included in the electrolyte.
In a particular embodiment of the foregoing (sometimes referred to as xe2x80x9cdesign IIxe2x80x9d herein), the solar cell comprises:
(a) a first substrate comprising a first electrode;
(b) a second substrate comprising a second electrode, with the first and second substrate being positioned to form a space therebetween, and with at least one of (i) the first substrate and the first electrode and (ii) the second substrate and the second electrode being transparent;
(c) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I:                               X          1                ⁢                              xe2x80x94            (                          X                              m                +                1                                      )                    m                                    (        I        )            
xe2x80x83wherein:
m is at least 1 (and typically two, three or four to twenty or more);
X1 is a charge separation group having an excited-state of energy equal to or lower than that of X2;
X2 through Xm+1 are chromophores;
X1 is electrically coupled to the first electrode; and
X1 through Xm+1 are selected so that, upon injection of either an electron or hole from X1 into the first electrode, the corresponding hole or electron from X1 is transferred to X2 (and optionally to X3, X4, and in some cases all the way to Xm+1); the solar cell further comprising
(d) an electrolyte in the space between the first and second substrates; and
(e) optionally, but preferably, a mobile charge carrier in the electrolyte.
In a currently preferred embodiment, X1 through Xm+1 are selected so that, upon injection of an electron from X1 into the first electrode, the corresponding hole from X1 is transferred to X2 through Xm+1.
Another particular embodiment (sometimes referred to as xe2x80x9cdesign IIIxe2x80x9d herein) of a solar cell as described above comprises:
(a) a first substrate comprising a first electrode;
(b) a second substrate comprising a second electrode, with the first and second substrate being positioned to form a space therebetween, and with at least one of (i) the first substrate and the first electrode and (ii) the second substrate and the second electrode being transparent;
(c) a layer of light harvesting rods electrically coupled to the first electrode, each of the light harvesting rods comprising a polymer of Formula I:                               X          1                ⁢                              xe2x80x94            (                          X                              m                +                1                                      )                    m                                    (        I        )            
xe2x80x83wherein:
m is at least 1 (and typically two, three or four to twenty or more);
X1 is a charge separation group having an excited-state of energy equal to or lower than that of X2;
X2 through Xm+1 are chromophores;
X1 is electrically coupled to the first electrode; and
Xm+1 is electrically coupled to the second electrode; the solar cell further comprising
(d) an electrolyte in the space between the first and second substrates.
Again, X1 through Xm+1 may be selected so that, upon injection of an electron or hole (preferably an electron) from X1 into the first electrode, the corresponding hole or electron from X1 is transferred to X2, or optionally to X3 or X4 or all the way through Xm+1.
A variety of different electrical devices comprised of a solar cell as described above having circuits (typically resistive loads) electrically coupled thereto can be produced with the solar cells of the invention, as discussed in greater detail below.
The present invention is explained in greater detail in the drawings herein and the specification set forth below.